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Preparation

Preheat oven to 350°F. Line 14 (1/2-cup) muffin cups with paper liners.
Melt unsweetened chocolate and 1/2 cup semisweet chunks in a metal bowl set over a pan of barely simmering water, stirring. Remove bowl from heat and cool slightly.
Whisk together flour, baking soda, and salt. Whisk vanilla into milk in another bowl.
Beat butter with brown sugar in a large bowl with an electric mixer until pale and fluffy, then add eggs 1 at a time, beating well after each addition. Add melted chocolate and beat well. Mix in flour and milk alternately in batches, scraping down side of bowl and beginning and ending with flour. Fold in remaining chunks.
Divide batter among lined muffin cups (fill empty cups halfway with water). Bake in upper and lower thirds of oven, switching position of pans halfway through baking, until a tester comes out clean, 22 to 25 minutes total. Cool muffins in pans on racks 15 minutes, then invert onto racks to cool.